Almost all its nearly 100 remaining reactors will be more than 60 years old by 2050. Their owners will have to decide whether the investments needed to keep them running are worth it, given the influx of cheap natural gas that has reshaped the U.S. energy economy.

So far, nuclear isn't winning. Vermont Yankee, which shut down Monday after 42 years of operation, is the fourth U.S. nuclear facility to close in two years. For the owners of each recent retiree—from Vermont Yankee to San Onofre in California, Kewaunee in Wisconsin, and Crystal River in Florida—the math just didn't work.

"When we looked at the cost of those improvements with what we projected as the cost of energy, the decision was that it would be better to shut the plant down," said Martin Cohn, spokesperson for Vermont Yankee's operator, Entergy.

More closures in the United States, the world's largest producer of nuclear power, could lead to a far different nuclear landscape from the one imagined before the gas boom. (Take the quiz: What Do You Know About Nuclear Power?)

The headwinds for nuclear also seem at odds with U.S. plans to address climate change by reducing greenhouse gas emissions. Nuclear power plants, which some environmentalists oppose because of safety concerns, emit no such heat-trapping gases.

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NG Maps Source: U.S. Nuclear Regulatory Commission

Too Dependent on Gas?

Yankee generated 70 percent of Vermont's electricity, but the state will be able to replace that power through other regional sources. Still, the shutdown has broader implications for New England. The share of power coming from natural gas-fired plants there leaped from 15 percent in 2000 to 46 percent in 2013, according to a recent report from regional grid operator New England ISO, which works to ensure reliability.

"The ISO is concerned about the amount of non-gas generators retiring, because it also increases the region's dependence on natural gas-fired generation," said spokeswoman Lacey Girard, noting that demand for natural gas has expanded faster than the pipeline capacity to deliver it. "It is a challenge for the region."

Natural gas, though, has much allure. Its ability to provide steady, inexpensive electric capacity has forced energy companies to take a second look at decades-old nuclear and coal plants. (See related story: "A Slew of Coal Plants Get New Lease on Life—With Gas.")

"So far, in all our research looking at the aging factors on nuclear plants, for the plants in the U.S., we have found no showstoppers," he said. "There are no aging mechanisms which aren't addressable."

High equipment repair costs led to the shutdown of two reactors at the San Onofre Nuclear Generating Station in California in 2013.

Photograph by David Bro, Corbis

Nuclear's Long-Term Legacy

Technically, the decades-long process of decommissioning a nuclear plant hasn't changed much over the years: The reactor is shut down, the radioactive fuel is removed and encased for storage, and the plant itself is eventually dismantled.

"We will have a concrete pad with a bunch of casks of spent fuel on it that will look almost exactly like the ones at Maine Yankee or Yankee Rowe [in Massachusetts]," which retired in 1996 and 1991, respectively, said Chris Wamser, Vermont Yankee site vice president.

The rules surrounding that process haven't changed either, which is a problem, according to Vermont Yankee executives. "Just because we shut down, we're still a nuclear plant with a license," said Barrett Green, who is leading the project. He said the NRC hasn't finalized a road map for decommissioning.

"We've had to make all sorts of petitions for amendments to the license, or exceptions to a rule, or various other allowances for the regulatory process to say it's OK for us to stop maintaining a system that we're never going to need again," Green said.

Outgoing Nuclear Regulatory Commission Chairman Allison Macfarlane acknowledged this issue at a National Press Club luncheon in November, suggesting that the agency has not prepared for a future where, instead of overseeing new nuclear plants, it is reckoning with older plants such as Vermont Yankee retiring ahead of schedule.

"The predicted 'nuclear renaissance' did not materialize," Macfarlane said. "It's time for the NRC to develop regulations specific to the decommissioning of nuclear power plants and to structure public expectations of the process."

Another pain point: The U.S. still has no federal repository for nuclear waste, a political football game seemingly longer than the half-life of uranium. The fight over plans to create such a repository under Yucca Mountain in Nevada has dragged on since the 1980s.

Instead, the waste is stored at the plants, where the owners are responsible for keeping it secure. Most of the decommissioning cost for Vermont Yankee after 2016 will go toward security measures and staff for the stored waste, Entergy's Cohn said.

Meanwhile, new nuclear development is haltingly under way. In 2012, the NRC approved licenses for the first new reactor construction in decades, but the projects—Vogtle in Georgia and V.C. Summer in South Carolina—have suffered delays and ballooning costs.

In a final rule issued this year, the NRC said that spent fuel could be stored safely at nuclear sites for 60 years or more. Timothy Frazier, a senior adviser at the Bipartisan Policy Center, which advocates for a federal waste site, says that decision contributed to a lack of urgency in Washington to hammer out a long-term solution to the nation's nuclear waste.

"It can sit for 60 years, or it can sit for 100 years," says Frazier, noting that spent nuclear fuel has to be isolated for hundreds of thousands of years as a potential hazard to people and the environment. "That's not the end of the problem."